Dye-sensitized solar cell, and electrode and laminated film for the same

ABSTRACT

A laminated film comprising a porous semiconductor layer, a transparent conductive layer and a transparent plastic film, wherein
         the porous semiconductor layer comprises crystalline titanium oxide fibers and crystalline titanium oxide fine particles, the crystalline titanium oxide fibers and the crystalline titanium oxide fine particles are substantially composed of an anatase phase and a rutile phase, the anatase phase content ratio calculated from the integral intensity ratio of X-ray diffraction is between 1.00 and 0.32, and the laminated film is used in an electrode for dye-sensitized solar cells,   and the electrode and a dye-sensitized solar cell comprising the same.

TECHNICAL FIELD

The present invention relates to a dye-sensitized solar cell, and anelectrode and a laminated film for the same. More specifically, itrelates to an electrode for dye-sensitized solar cells which can be usedto manufacture a dye-sensitized solar cell having high photovoltaicpower generation performance in spite of the use of a plastic substrate,a laminated film for the same, and the dye-sensitized solar cell.

BACKGROUND ART

A dye-sensitized solar cell has been attracting attention as a new solarcell substituting a silicon-based solar cell since a photoelectricconversion element comprising dye-sensitized semiconductor fineparticles was proposed (Nature, vol. 353, p. 737 to 740 (1991)).

A dye-sensitized solar cell comprising a plastic substrate attractsattention because it can be made soft and lightweight. In the case of adye-sensitized solar cell comprising a glass substrate which is commonlyused, a high-temperature heat treatment is carried out to form a porousstructure so as to enhance integrity among oxide semiconductor particlesand improve photoelectric conversion efficiency. However, thetemperature is generally 400° C. or higher, and it is difficult to carryout a high-temperature heat treatment directly on the plastic substrate.To cope with this, in JP-A 11-288745, a dye-sensitized solar cellcomprising a plastic substrate is manufactured by oxidizing metal foiland making the surface of the metal foil uneven. However, the specificsurface area of the dye-sensitized solar cell is not sufficiently large,whereby photoelectric conversion efficiency is not fully improved. InJP-A 2001-160426, after the high-temperature heat treatment of a metaloxide is carried out on metal foil, the metal oxide layer is removed andfixed on the plastic substrate by a binder. However, this process iscomplicated and not suitable for mass-production. In JP-2002-50413,metal oxide particles are coated on a plastic substrate to form asemiconductor metal oxide layer. However, the metal oxide particlesfixed on a transparent conductive layer fall off in a powdery form atthe time of handling or peel off in an electrolyte.

JP-A 2001-93590 and JP-A 2001-358348 disclose that a metal oxideneedle-like crystal is used as an electrode for solar cells to improvecharge transport efficiency. However, to attain high charge transportefficiency by obtaining a good porous structure, the crystal state ofthe metal oxide must be properly controlled. For example, in the case oftitanium oxide as the metal oxide, an anatase phase is preferred.However, it is difficult to manufacture needle-like titanium oxidehaving an anatase phase, and titanium oxide having a more stable rutilephase is first formed. As a result, photoelectric conversion efficiencyis not satisfactory.

Meanwhile, electrospinning is one of the methods of manufacturing ametal oxide. In this method, an oxide precursor containing a burned-outcomponent such as a polymer is ejected onto a substrate at a high aspectratio and heated at a high temperature to obtain a metal oxide. Anelectrode for dye-sensitized solar cells having a metal oxide layer on aglass substrate by using this electrospinning has already been known.The above dye-sensitized solar cell is described in US2005/0109385 and“Nanotechnology” written by Mi Yeon Songs et al., p. 1861 to 1865, 2004.

In the above-described electrode for dye-sensitized solar cells, a metaloxide precursor is ejected onto a transparent conductive layer overlyinga glass substrate at a high aspect ratio to be deposited and baked at ahigh temperature to obtain a metal oxide layer. The metal oxide tends topeel off from the transparent conductive layer due to the shrinkage ofthe metal oxide at the time of baking. Even when the metal oxide layeris formed by electrospinning, a sufficiently large specific surface areaand sufficiently high charge transport efficiency cannot be obtained.Since the step of baking the metal oxide over the glass substrate iscarried out at 400° C. or higher, it is difficult to apply thistechnology to an electrode for dye-sensitized solar cells whichcomprises a plastic substrate.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an electrode fordye-sensitized solar cells which can obtain high charge transportefficiency by adsorbing a sufficient amount of a dye in spite of the useof a plastic substrate, comprises a porous oxide film formed on asubstrate with high adhesion without peeling off from the substrate, andcan be used to manufacture a dye-sensitized solar cell having highphotovoltaic power generation performance.

It is another object of the present invention to provide a laminatedfilm comprising a plastic substrate for the above electrode.

It is still another object of the present invention to provide adye-sensitized solar cell comprising the above electrode.

The other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a laminated filmwhich comprises a porous semiconductor layer, a transparent conductivelayer and a transparent plastic film, wherein

the porous semiconductor layer comprises crystalline titanium oxidefibers and crystalline titanium oxide fine particles, the crystallinetitanium oxide fibers and the crystalline titanium oxide fine particlesare substantially composed of an anatase phase and a rutile phase, theanatase phase content ratio calculated from the integral intensity ratioof X-ray diffraction is between 1.00 and 0.32, and the laminated film isused in an electrode for dye-sensitized solar cells.

According to the present invention, secondly, the above objects andadvantages of the present invention are attained by an electrode fordye-sensitized solar cells which comprises the above laminated film anda dye adsorbed to the porous semiconductor layer of the laminated film.

According to the present invention, thirdly, the above objects andadvantages of the present invention are attained by a dye-sensitizedsolar cell comprising the above electrode of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining an ejection apparatus used inelectrospinning employed in Examples.

EXPLANATIONS OF THE LETTERS OR NOTATIONS

-   -   1. solution spray nozzle    -   2. solution    -   3. solution storage tank    -   4. electrode    -   5. high-voltage generator

BEST MODE FOR CARRYING OUT THE INVENTION

In the laminated film of the present invention, a porous semiconductorlayer comprises crystalline titanium oxide fibers and crystallinetitanium oxide fine particles. Since the porous semiconductor layercomprises crystalline titanium oxide fibers and crystalline titaniumoxide fine particles, both an excellent porous structure and a largespecific surface area can be obtained. The crystalline titanium oxidefibers and the crystalline titanium oxide fine particles aresubstantially composed of an anatase phase and a rutile phase, and thearea ratio in X-ray diffraction of the anatase phase to the total of theanatase phase and the rutile phase of the porous semiconductor layercomprising the crystalline titanium oxide fibers and the crystallinetitanium oxide fine particles is between 1.00 and 0.32. The area ratiosubstantially does not exceed 1.0. When it is less than 0.32, it isdifficult to attain high charge transport efficiency disadvantageously.

The anatase phase content ratio calculated from the integral intensityratio of X-ray diffraction is obtained from the following equation byestimating integral intensity IA (anatase phase) and integral intensityIR (rutile phase) from diffraction peaks derived from the anatase phaseand rutile phase titanium oxides which appear at 2θ=25.3° and 27.4°,respectively, in an X-ray profile obtained by carrying out intensitycorrection.Anatase phase content ratio=IA/(IA+IR)

The expression “substantially composed of an anatase phase and a rutilephase” means that the ratio of the total of the anatase phase and therutile phase to all the integral intensity in X-ray diffraction ispreferably 80% or more, more preferably 83% or more, particularlypreferably 88% or more. If the crystalline titanium oxide fibers and thecrystalline titanium oxide fine particles are not substantially composedof the anatase phase and the rutile phase, charge transport efficiencywill become unsatisfactory disadvantageously.

In the present invention, the average crystallite size measured by theX-ray diffraction of the anatase phase in the porous semiconductor layeris preferably 10 to 100 nm, more preferably 20 to 100 nm. When theaverage crystallite size is smaller than 10 nm, the number of interfacesbetween crystals increases, thereby reducing charge transport efficiencydisadvantageously. When the size is larger than 100 nm, the specificsurface area of the porous semiconductor layer decreases, whereby asufficient amount of generated electricity is not obtaineddisadvantageously.

The measurement of the average crystallite size is carried out by X-raydiffraction. For the measurement of X-ray diffraction, a reflectionmethod was employed by using the ROTA FLEX RU200B of Rigaku Denki Co.,Ltd. and a goniometer having a radius of 185 nm, and CuKα raymonochromized with a monochrometer was used as the X-ray. A measurementsample obtained by adding X high-purity silicon powders for X-raydiffraction standards as an internal standard to the obtained poroussemiconductor was used.

Intensity correction was made on the X-ray diffraction profile obtainedabove, and the diffraction angle 2θ was corrected with the 111diffraction peak of the internal standard silicon. The half-value widthof the 111 diffraction peak of silicon was 0.15° or less. Thecrystallite size was calculated from the following Scherrer's equationby using diffraction peaks which appeared at around 25.3° in thecorrected X-ray diffraction profile. The diffraction peaks of titaniumoxide and silicon at 2θ=24 to 30° were derived from Cu Kα1 and Cu Kα2rays, not separate from each other and all handled as being derived fromCu Kα.D=K×λ/β cos θD: crystallite size (nm)λ: measurement X-ray wavelength (nm)β: expansion of diffraction line by crystallite sizeθ: Bragg angle of diffraction peak (°)K: form factor (Scherrer's constant).

Since β corrects the expansion of an optical system, (β=B−b) obtained bysubtracting the half-value width “b” of the 111 diffraction peak of theinternal standard silicon from the half-value width “B” of thediffraction peak of titanium oxide which appeared at around 25.3° wasused, K=1 and λ=0.15418 nm.

In the present invention, the above porous semiconductor layer ispreferably formed by applying a dispersion (coating) obtained bydispersing the crystalline titanium oxide fibers and the crystallinetitanium oxide fine particles in a dispersion medium to the transparentconductive layer of a transparent plastic film having the transparentconductive layer, or by adding the crystalline titanium oxide fineparticles to the crystalline titanium oxide fibers in a nonwoven stateto form a layer.

As the dispersion medium of the dispersion (coating), for example, wateror an organic solvent may be used, and an alcohol is preferably used asthe organic solvent. For dispersion into the dispersion medium, adispersion aid may be added in a small amount as required. Examples ofthe dispersion aid include surfactants, acids and chelating agents.

To improve adhesion between the crystalline titanium oxide fibers andthe crystalline titanium oxide fine particles, a binder may be used.

<Crystalline Titanium Oxide Fibers>

The crystalline titanium oxide fibers are preferably manufactured byelectrospinning.

In the electrospinning, the crystalline titanium oxide fibers can beobtained by ejecting a solution comprising a mixture of a titanium oxideprecursor and a compound which forms a complex with the titanium oxideprecursor, a solvent and a solute having high aspect formability onto acollection substrate to deposit it on the substrate and baking it.

Examples of the titanium oxide precursor include titaniumtetramethoxide, titanium tetraethoxide, titanium tetranormalpropoxide,titanium tetraisopropoxide, titanium tetranormalbutoxide and titaniumtetratertiarybutoxide. Titanium tetraisopropoxide and titaniumtetranormalbutoxide are preferred because they are easily acquired.

Examples of the compound forming a complex with the titanium oxideprecursor include coordination compounds such as carboxylic acids,amides, esters, ketones, phosphines, ethers, alcohols and thiols. Out ofthese, acetylacetone, acetic acid and tetrahydrofuran are preferablyused. The amount of the compound forming a complex with the titaniumoxide precursor is 0.5 equivalent or more, preferably 1 to 10equivalents based on the titanium oxide precursor.

Examples of the solvent include aliphatic hydrocarbons such as hexane;aromatic hydrocarbons such as toluene and tetralin; alcohols such asn-butanol and ethylene glycol; ethers such as tetrahydrofuran anddioxane; dimethyl sulfoxide, N,N-dimethylformamide,n-methylaminopyridine and water. Out of these, N,N-dimethylformamide andwater are preferred from the viewpoint of affinity for solutes. Thesolvents may be used alone or in combination. The amount of the solventis preferably 0.5 to 30 times, more preferably 0.5 to 20 times based onthe weight of the titanium oxide precursor.

An organic polymer is preferably used as the solute having high aspectratio formability because it must be removed by baking from theviewpoint of handling. Examples of the solute include polyethyleneoxide, polyvinyl alcohol, polyvinyl ester, polyvinyl ether, polyvinylpyridine, polyacrylamide, ether cellulose, pectine, starch, polyvinylchloride, polyacrylonitrile, polylactic acid, polyglycolic acid,polylactic acid-polyglycolic acid copolymer, polycaprolactone,polybutylene succinate, polyethylene succinate, polystyrene,polycarbonate, polyhexamethylene carbonate, polyallylate, polyvinylisocyanate, polybutyl isocyanate, polymethyl methacrylate, polyethylmethacrylate, polynormalpropyl methacrylate, polynormalbutylmethacrylate, polymethyl acrylate, polyethyl acrylate, polybutylacrylate, polyethylene terephthalate, polytrimethylene terephthalate,polyethylene naphthalate, polyparaphenylene terephthalamide,polyparaphenylene terephthalamide-3,4′-oxydiphenylene terephthalamidecopolymer, polymetaphenylene isophthalamide, cellulose diacetate,cellulose triacetate, methyl cellulose, propyl cellulose, benzylcellulose, fibroin, natural rubber, polyvinyl acetate, polyvinylmethylether, polyvinylethyl ether, polyvinylnormalpropyl ether,polyvinylisopropyl ether, polyvinylnormalbutyl ether, polyvinylisobutylether, polyvinyltertiarybutyl ether, polyvinylidene chloride,poly(N-vinylpyrrolidone), poly(N-vinylcarbazol), poly(4-vinylpyridine),polyvinylmethyl ketone, polymethylisopropenyl ketone, polypropyleneoxide, polycyclopentene oxide, polystyrene sulfone, nylon 6, nylon 66,nylon 11, nylon 12, nylon 610, nylon 612 and copolymers thereof. Out ofthese, polyacrylonitrile, polyethylene oxide, polyvinyl alcohol,polyvinyl acetate, poly(N-vinylpyrrolidone), polylactic acid, polyvinylchloride and cellulose triacetate are preferred from the viewpoint ofsolubility in the solvent.

When the molecular weight of the organic polymer is too low, the amountof the organic polymer becomes large and the amount of a gas generatedby baking becomes large, whereby it is fairly possible that a defect isproduced in the structure of the metal oxide. Therefore, the molecularweight of the organic polymer is suitably set. The molecular weight ispreferably 100,000 to 8,000,000, more preferably 100,000 to 600,000 inthe case of polyethylene glycol out of polyethylene oxides.

The amount of the solute having high aspect ratio formability ispreferably as small as possible from the viewpoint of the improvement ofthe denseness of titanium oxide if the concentration at which a highaspect ratio is formed can be obtained, specifically 0.1 to 200 wt %,more preferably 1 to 150 wt % based on the weight of the titanium oxideprecursor.

Electrospinning is a known technique for obtaining a titanium oxideejected product by ejecting a solution of a matrix having high aspectratio formability into an electrostatic field formed between electrodes,spinning the solution toward the electrodes and collecting the formedhigh aspect ratio formed product on a collection substrate. The titaniumoxide ejected product maintains a high aspect ratio even when it is alaminate obtained after the solvent for dissolving the matrix havinghigh aspect ratio formability is distilled off or even when the solventis contained in the ejected product.

Although electrospinning is generally carried out at room temperature,when the volatilization of the solvent is insufficient, the temperatureof the spinning atmosphere or the temperature of the collectionsubstrate may be controlled as required.

Any metal, inorganic or organic electrodes may be used as the electrodesfor electrospinning as long as they show conductivity, or electrodeshaving a metal, inorganic or organic thin film having conductivity on aninsulating substance may also be used.

The electrostatic field is formed between a pair or a plurality ofelectrodes, and high voltage may be applied to all of the electrodes.This includes a case where three electrodes consisting of twohigh-voltage electrodes which differ in voltage value (for example, 15kV and 10 kV) and an electrode connected to an earth are used and a casewhere more than 3 electrodes are used.

The high aspect ratio titanium oxide ejected product which is ejected byelectrospinning is collected on an electrode which is a collectionsubstrate. Then, this titanium oxide ejected product is baked. Althougha general electric furnace may be used for baking, an electric furnacewhose inside gas can be substituted as required may be used. The bakingtemperature at which crystals grow fully and crystal growth can becontrolled is preferred. In order to control the growth of anatase phasecrystals and rutile phase crystal transition, the baking temperature ispreferably 300 to 900° C., more preferably 500 to 800° C. Thecrystalline titanium oxide fibers obtained as described above preferablyhave the following properties.

The fiber diameter is 50 to 1,000 nm, and the fiber length/fiberdiameter ratio is 5 or more, preferably 5 to 300. When the fiberdiameter is smaller than 50 nm, it is substantially difficult to handlethe fibers disadvantageously. When the fiber diameter is larger than1,000 nm, a dye cannot be adsorbed to the surface of each fiber fullyand electricity is not fully generated disadvantageously.

The (anatase phase)/(anatase phase+rutile phase) ratio obtained from thearea ratio of the crystal phases in X-ray diffraction is 1.00 to 0.50.When the ratio is lower than 0.50, charge transport efficiency lowersdisadvantageously.

The size of the anatase phase crystallite in X-ray diffraction is 10 to200 nm. When the size is smaller than 10 nm, charge transport efficiencylowers disadvantageously. When the size is larger than 200 nm, thespecific surface area of the porous semiconductor layer decreases,thereby making it impossible to obtain a sufficient amount of generatedelectricity disadvantageously.

The BET specific surface area is 0.1 to 1,000 m²/g. When the specificsurface area is smaller than 0.1 m²/g, the obtained fibers cannot adsorba dye fully and electricity is not fully generated disadvantageously.When the specific surface area is larger than 1,000 m²/g, it issubstantially difficult to handle the fibers disadvantageously.

<Crystalline Titanium Oxide Fine Particles>

Meanwhile, the crystalline titanium oxide fine particles have a particlediameter of preferably 2 to 500 nm, more preferably 5 to 200 nm. Whenthe particle diameter is smaller than 2 nm, the number of particleinterfaces increases and charge transport efficiency lowersdisadvantageously. When the particle diameter is larger than 500 nm, theamount of the adsorbed dye decreases and a sufficient amount ofgenerated electricity is not obtained disadvantageously.

The crystal type of the titanium oxide fine particles may be anatase orrutile, and the titanium oxide fine particles may be used as a mixtureof these crystal types.

<Formation of Porous Semiconductor Layer>

Preferably, the porous semiconductor layer contains 10 wt % or more ofthe crystalline titanium oxide fibers and 15 wt % or more of thecrystalline titanium oxide fine particles. When the content of thecrystalline titanium oxide fibers is lower than 10 wt %, sufficientlyhigh porosity is not obtained disadvantageously and when the content ofthe crystalline titanium oxide fine particles is lower than 15 wt %, asufficiently amount of generated electricity is not obtaineddisadvantageously. The contents of the crystalline titanium oxide fibersand the crystalline titanium oxide fine particles are more preferably 15to 80 wt % and 20 to 85 wt %, respectively.

To form the porous semiconductor layer, there may be employed a methodin which a coating solution comprising the crystalline titanium oxidefibers and the crystalline titanium oxide fine particles dispersedtherein is applied or a method in which the crystalline titanium oxidefine particles are added to the crystalline titanium oxide fibers in anon-woven state to form a layer.

The solid content of the dispersion used when the porous semiconductorlayer is formed by applying a coating solution comprising thecrystalline titanium oxide fibers and the crystalline titanium oxidefine particles dispersed therein is preferably 1 to 80 wt %. When thesolid content of the dispersion is lower than 1 wt %, the final poroussemiconductor layer becomes thin disadvantageously. When the solidcontent is higher than 80 wt %, the viscosity becomes too high, therebymaking it difficult to apply the solution disadvantageously. The contentis more preferably 4 to 60 wt %.

The coating solution which is a dispersion can be prepared by dispersingthe crystalline titanium oxide fibers and the crystalline titanium oxidefine particles in a dispersion medium. In the dispersion medium, theymay be dispersed by physical dispersion using a ball mill, mediumagitation mill or homogenizer, or an ultrasonic treatment. Thedispersion medium for the dispersion is, for example, water or anorganic solvent, and an alcohol is preferably used as the organicsolvent.

A binder for the titanium oxide fine particles may be added to thisdispersion. A titanium oxide precursor may be preferably used as thebinder. Example of the binder include titanium tetramethoxide, titaniumtetraethoxide, titanium tetranormalpropoxide, titaniumtetraisopropoxide, titanium tetranormalbutoxide, titaniumtetratertiarybutoxide and hydrolysates of these titanium oxideprecursors. They may be used alone or in combination.

To apply the coating solution to a transparent conductive layer formedon a transparent plastic film, any means which is commonly used forapplication may be employed. For example, roller coating, dip coating,air knife coating, blade coating, wire bar coating, slide hoppercoating, extrusion coating or curtain coating may be used. Spin coatingor spray coating with a general-purpose machine may also be used. Threebig printing techniques which are relief printing, offset printing andgravure printing and wet printing techniques such as intaglio printing,rubber printing and screen printing may be used. Out of these, apreferred film forming method may be selected according to the viscosityand wet thickness of the solution. The coating weight of the coatingsolution is preferably 0.5 to 20 g, more preferably 5 to 10 g per 1 m²of the substrate at the time of drying.

After the coating solution is applied to the transparent conductivelayer, it is heated to form a porous semiconductor layer. This heattreatment may be carried out in the drying step or another step afterthe drying step. The heat treatment is preferably carried out at 100 to250° C. for 1 to 120 minutes, more preferably at 150 to 230° C. for 1 to90 minutes, particularly preferably 180 to 220° C. for 1 to 60 minutes.This heat treatment makes it possible to suppress a rise in theresistance of the porous semiconductor layer while the thermaldeformation of the film supporting the transparent conductive layer isprevented. The final thickness of the porous semiconductor layer ispreferably 1 to 30 μm, more preferably 2 to 10 μm, particularlypreferably 2 to 6 μm to enhance transparency.

A treatment for reinforcing physical bonding between particles, such asthe application of ultraviolet light that the titanium oxide fineparticles constituting the porous semiconductor layer strongly absorb orthe application of microwaves to heat the fine particle layer may becarried out.

The method in which the crystalline titanium oxide fine particles areadded to the crystalline titanium oxide fibers in a nonwoven stateformed on the transparent conductive layer overlying the transparentplastic film to form a layer can be carried out by using pressurebonding or thermal pressure bonding by means of a press or a roll,bonding by means of a binder or a combination thereof.

In the case of the thermal pressure bonding, preferably, the surfaces ofthe crystalline titanium oxide fibers in a nonwoven state or the surfaceof the transparent conductive layer is activated to improve adhesion. Asmeans of activation, the surfaces of the crystalline titanium oxidefibers in a nonwoven state are activated with an acid or alkalinesolution, the surface of a thin film is activated by applyingultraviolet radiation or electron beams, or the surface is activated bycarrying out a corona treatment or plasma treatment. Preferably, thesurface is activated with an acid or alkaline solution or by carryingout a plasma treatment.

When a binder is used, a binder which does not prevent charge migration,such as a metal oxide or a precursor thereof, a conductive polymer, aconductive inorganic material, an organic adhesive, preferably a metaloxide or a precursor thereof may be used. As for bonding by means of abinder, a binder or a dispersion of a binder is applied to thetransparent conductive layer or the crystalline titanium oxide fibers ina nonwoven state for bonding, or the crystalline titanium oxide fibersin a nonwoven state are placed on the transparent conductive layer andthen a binder or a dispersion of a binder is added.

The crystalline titanium oxide fine particles are added to thecrystalline titanium oxide fibers in a nonwoven state to form the poroussemiconductor layer. To add the crystalline titanium oxide fineparticles, a method in which the crystalline titanium oxide fibers in anonwoven state are impregnated with a dispersion containing thecrystalline titanium oxide fine particles and then heated, a method inwhich a dispersion containing the crystalline titanium oxide fineparticles is applied to the transparent conductive layer, thecrystalline titanium oxide fibers in a nonwoven state, or both of thecrystalline titanium oxide fibers in a nonwoven state and thetransparent conductive layer with a spray or a bar coater, a method inwhich the crystalline titanium oxide fibers in a nonwoven state and thecrystalline titanium oxide fine particles are thermally pressure bondedtogether, a method in which the crystalline titanium oxide fibers in anonwoven state and the crystalline titanium oxide fine particles aretreated, for example, in an autoclave to be bonded together, a method inwhich fine particles are hydrothermally synthesized in the presence ofthe crystalline titanium oxide fibers in a nonwoven state and a metaloxide precursor, a method in which fine particles are formed by electronbeams or UV treatment in the presence of the crystalline titanium oxidefibers in a nonwoven state and a metal oxide precursor, or a method inwhich the crystalline titanium oxide fine particles are bonded to thecrystalline titanium oxide fibers in a nonwoven state by sputtering maybe employed. These methods may be used in combination.

Out of these, the method in which the crystalline titanium oxide fibersin a nonwoven state are impregnated with a dispersion containing thecrystalline titanium oxide fine particles and then heated and the methodin which a dispersion containing the crystalline titanium oxide fineparticles is applied to the transparent conductive layer, thecrystalline titanium oxide fibers in a nonwoven state, or both of thecrystalline titanium oxide fibers in a nonwoven state and thetransparent conductive layer with a spray or a bar coater, arepreferred. When these methods are employed, the crystalline titaniumoxide fine particles can be charged into the inside easilyadvantageously. To improve adhesion between the crystalline titaniumoxide fine particles and the crystalline titanium oxide fibers in anonwoven state, the above-described surface activation may be employedor a binder may be used.

The impregnation or application of the crystalline titanium oxide fineparticles to the crystalline titanium oxide fibers in a nonwoven statemay be carried out before, after or at the same time when thecrystalline titanium oxide fibers in a nonwoven state are placed on thetransparent conductive layer. When the dispersion is used, bonding tothe transparent conductive layer and the addition of the fine particlescan be carried out simultaneously by adding the binder to thedispersion.

For example, a titanium oxide precursor can be preferably used as theabove binder. Examples of the titanium oxide precursor include titaniumtetramethoxide, titanium tetraethoxide, titanium tetranormalpropoxide,titanium tetratertiarybutoxide and hydrolysates of these titanium oxideprecursors. They may be used alone or in combination.

When the crystalline titanium oxide fine particles are added in the formof a dispersion, the amount of the crystalline titanium oxide fineparticles used in the dispersion is preferably 0.05 to 90 wt %, morepreferably 1 to 70 wt %, particularly preferably 1 to 50 wt %. When theamount of the crystalline titanium oxide fine particles is smaller than0.05 wt %, the final porous semiconductor layer becomes thindisadvantageously. When the amount is larger than 90 wt %, the viscositybecomes too high, thereby making it difficult to apply the dispersiondisadvantageously.

As the dispersion medium of the dispersion is used water or an organicsolvent, and an alcohol is preferably used as the organic solvent. Todisperse the crystalline titanium oxide fine particles into thedispersion medium, a dispersion aid may be added in a small amount asrequired. Examples of the dispersion aid include surfactants, acids andchelating agents.

To improve charge transport efficiency in the porous semiconductorlayer, it is preferred that the crystalline titanium oxide fineparticles should be added to the crystalline titanium oxide fibers in anonwoven state formed on the transparent conductive layer overlying thetransparent plastic film to form a layer and then heated. This heattreatment may be carried out in the drying step or another step afterthe drying step. The heat treatment is preferably carried out at 100 to250° C. for 1 to 120 minutes, more preferably at 150 to 230° C. for 1 to90 minutes, particularly preferably at 180 to 220° C. for 1 to 60minutes. This heat treatment makes it possible to suppress a rise in theresistance of the porous semiconductor layer while the thermaldeformation of the plastic substrate is prevented.

A treatment for reinforcing physical bonding between metal oxides byapplying ultraviolet light that the metal oxides strongly absorb ormicrowaves to the porous semiconductor layer to which the metal oxidefine particles have been added to heat the metal oxides may be carriedout.

To prevent an electric short-circuit between the transparent conductivelayer carrying the porous semiconductor and a counter electrode, in allthe methods of forming the porous semiconductor layer, a primer layermay be formed on the transparent conductive layer. The primer layer ispreferably made of TiO₂, SnO₂, ZnO or Nb₂O₅, particularly preferablyTiO₂. This primer layer may be formed by spray-pyrolysis described inElectrochim. Acta 40, 643-652 (1995) or sputtering. The thickness ofthis primer layer is preferably 5 to 1,000 nm, more preferably 10 to 500nm.

<Transparent Plastic Film>

In the present invention, a plastic film is used as a substrate forsupporting the transparent conductive layer. The plastic film ispreferably a polyester film, and the polyester constituting thispolyester film is a linear saturated polyester which is synthesized froman aromatic dibasic acid or an ester forming derivative thereof and adiol or an ester forming derivative thereof.

Examples of the polyester include polyethylene terephthalate,polyethylene isophthalate, polybutylene terephthalate,poly(1,4-cyclohexylene dimethylene terephthalate) andpolyethylene-2,6-naphthalate. It may be a copolymer of these or a blendof one of these and a small amount of another resin. Out of thesepolyesters, polyethylene terephthalate and polyethylene-2,6-naphthalateare preferred because they have good balance among mechanical propertiesand optical properties.

Particularly polyethylene-2,6-naphthalate is most preferred because itis superior to polyethylene terephthalate in mechanical strength, heatshrinkage factor and the amount of an oligomer produced at the time ofheating.

The polyethylene terephthalate comprises an ethylene terephthalate unitin an amount of preferably 90 mol % or more, more preferably 95 mol % ormore, particularly preferably 97 mol % or more. Thepolyethylene-2,6-naphthalate comprises a polyethylene-2,6-naphthalateunit in an amount of preferably 90 mol % or more, more preferably 95 mol% or more, particularly preferably 97 mol % or more. The polyester maybe a homopolymer or a copolymer comprising a third component, preferablya homopolymer.

The intrinsic viscosity of the polyester is preferably 0.40 dl/g ormore, more preferably 0.40 to 0.90 dl/g. When the intrinsic viscosity islower than 0.40 dl/g, a process break may occur frequentlydisadvantageously and when the intrinsic viscosity is higher than 0.90dl/g, melt extrusion becomes difficult due to high melt viscosity withthe result that the polymerization time becomes long uneconomically.

The polyester can be obtained by a conventionally known method. Forexample, a polyester having a low degree of polymerization can beobtained directly through a reaction between a dicarboxylic acid and aglycol. It may also be obtained by carrying out a polymerizationreaction in the presence of a polymerization reaction catalyst after anester interchange reaction between a lower alkyl ester of a dicarboxylicacid and a glycol. As the ester interchange reaction catalyst used is aconventionally known compound containing sodium, potassium, magnesium,calcium, zinc, strontium, titanium, zirconium, manganese or cobalt.Examples of the polymerization reaction catalyst include conventionallyknown catalysts such as antimony compounds including antimony trioxideand antimony pentoxide, germanium compounds typified by germaniumdioxide, and titanium compounds including tetraethyl titanate,tetrapropyl titanate, tetraphenyl titanate and partial hydrolysatesthereof, and titanylammonium oxalate, titanylpotassium oxalate andtitanium trisacetyl acetonate. When polymerization is carried outthrough an ester interchange reaction, a phosphorus compound such astrimethyl phosphate, triethyl phosphate, tri-n-butyl phosphate ororthophosphoric acid is generally added to deactivate the esterinterchange catalyst before the polymerization reaction. The content ofelemental phosphorus in the polyester is preferably 20 to 100 ppm fromthe viewpoint of heat stability. The polyester may be processed into achip after melt polymerization, and solid-phase polymerization may befurther carried out under a heated reduced pressure or in a inert gasstream such as nitrogen stream.

The polyester film preferably contains substantially no particles. Whenit contains particles, its high transparency may be impaired or itssurface may become rough to make it difficult to process the transparentconductive layer. The haze value of the film is preferably 1.5% or less,more preferably 1.0% or less, particularly preferably 0.5% or less.

The polyester film has a light transmittance at a wavelength of 370 nmof preferably 3% or less and at a wavelength of 400 nm of 70% or more.The light transmittance is a numerical value measured by using theMPC3100 spectrophotometer of Shimadzu Corporation. This lighttransmittance can be obtained by using a polyester containing a monomerwhich absorbs ultraviolet light, such as 2,6-naphthalenedicarboxylicacid, or by containing an ultraviolet absorber in the polyester.

Examples of the ultraviolet absorber include cyclic iminoester compoundssuch as 2,2′-p-phenylenebis(3,1-benzoxazin-4-one),2,2′-p-phenylenebis(6-methyl-3,1-benzoxazin-4-one),2,2′-p-phenylenebis(6-chloro-3,1-benzoxazin-4-one),2,2′-(4,4′-diphenylene)bis(3,1-benzoxazin-4-one) and2,2′-(2,6-naphthylene)bis(3,1-benzoxazin-4-one).

The polyester film has a 3D center line average roughness on both sidesof preferably 0.0001 to 0.02 μm, more preferably 0.0001 to 0.015 μm,particularly preferably 0.0001 to 0.010 μm. Particularly when the 3Dcenter line average roughness of at least one side is 0.0001 to 0.005μm, the transparent conducive layer can be easily processedadvantageously. The most preferred surface roughness of at least oneside is 0.0005 to 0.004 μm.

The thickness of the polyester film is preferably 10 to 500 μm, morepreferably 20 to 400 μm, particularly preferably 50 to 300 μm.

A description is subsequently given of the preferred method ofmanufacturing the polyester film. The glass transition temperature isabbreviated as Tg. The polyester film can be obtained by melt extrudinga polyester into a film form, cooling it on a casting drum to solidifyit so as to obtain an unstretched film, stretching this unstretched filmat a total draw ratio of 3 to 6 times in a longitudinal direction onceor twice or more at Tg to (Tg+60)° C. and then at a draw ratio of 3 to 5times in a transverse direction at Tg to (Tg+60)° C., and optionallyheat setting the stretched film at Tm180 to 255° C. for 1 to 60 seconds.To reduce the difference in heat shrinkage factor between thelongitudinal direction and the transverse direction of the polyesterfilm and heat shrinkage in the longitudinal direction, a method in whichthe film is shrunk in the longitudinal direction by a heat treatment asshown in JP-A 57-57628 or a method in which the film is relaxed andheated in a suspended state as shown in JP-A 1-275031 may be employed.

<Transparent Conductive Layer>

As the transparent conductive layer may be used a thin film of aconductive metal oxide (fluorine doped tin oxide, indium-tin compositeoxide (ITO) or indium-zinc composite oxide (IZO)), a thin film of ametal such as platinum, gold, silver, copper or aluminum, or a carbonmaterial film. This transparent conductive layer may be a laminateconsisting of two or more layers or a composite film. Out of these, ITOand IZO thin films are particularly preferred because they have a highlight transmittance and low resistance.

The surface resistance of the transparent conductive layer is preferably100Ω/□ or less, more preferably 40Ω/□ or less. When the surfaceresistance is higher than 100Ω/□, the inside resistance of the cellbecomes too high, whereby photovoltaic power generation efficiencylowers disadvantageously.

The thickness of the transparent conductive layer is preferably 100 to500 nm. When the thickness is smaller than 100 nm, the surfaceresistance value cannot be reduced to the full and when the thickness islarger than 500 nm, light transmittance lowers and the transparentconductive layer is apt to be broken disadvantageously.

The surface tension of the transparent conductive layer is preferably 40mN/m or more, more preferably 65 mN/m or more. When the surface tensionis lower than 40 mN/m, adhesion between the transparent conductive layerand the porous semiconductor may deteriorate and when the surfacetension is 65 mN/m or more, the formation of the porous semiconductorlayer by the application of an aqueous coating solution containing wateras the main component of a solvent becomes easy advantageously.

The transparent conductive layer having the above properties can beobtained by forming a transparent conductive layer from ITO or IZO andthen carrying out processing by any one of the following methods.

(1) A method of activating the surface of the transparent conductivelayer with an acid or alkaline solution

(2) A method of activating the surface of the transparent conductivelayer by applying ultraviolet radiation or electron beams

(3) A method of activating the surface of the transparent conductivelayer by carrying out a corona treatment or a plasma treatment

Out of these, the method of activating the surface by a plasma treatmentis particularly preferred because high surface tension is obtained.

<Adhesive Layer>

To improve adhesion between the polyester film and the transparentconductive layer, an adhesive layer may be formed between the polyesterfilm and the transparent conductive layer. The thickness of the adhesivelayer is preferably 10 to 200 nm, more preferably 20 to 150 nm. When thethickness of the adhesive layer is smaller than 10 nm, the effect ofimproving adhesion becomes unsatisfactory and when the thickness islarger than 200 nm, the cohesion failure of the adhesive layer is apt tooccur and adhesion may deteriorate disadvantageously.

To form the adhesive layer, it is preferably formed by coating in thecourse of manufacturing the polyester film and more preferably formed onthe polyester film before the completion of orientation crystallization.The “polyester film before the completion of orientationcrystallization” includes an unstretched film, a monoaxially orientedfilm obtained by stretching an unstretched film in one of thelongitudinal direction and the transverse direction, and an orientedfilm obtained by stretching an unstretched film in both the longitudinaldirection and the transverse direction at a low draw ratio (biaxiallyoriented film before the completion of orientation crystallization byre-stretching in the longitudinal direction or the transverse directionin the end). Preferably, the unstretched film or the monoaxiallyoriented film out of these is preferably coated with an aqueous coatingsolution of the above composition, stretched in the longitudinaldirection and/or the transverse direction and heat set.

The adhesive layer is preferably made of a material having excellentadhesion to both of the polyester film and the transparent conductivelayer, as exemplified by polyester resin, acrylic resin, urethaneacrylic resin, silicon acrylic resin, melamine resin and polysiloxaneresin. These resins may be used alone or in combination of two or more.

<Hard Coat Layer>

To improve adhesion, especially the durability of adhesion between thepolyester film and the transparent conductive layer, a hard coat layermay be formed between the adhesive layer and the transparent conductivelayer. The thickness of the hard coat layer is preferably 0.01 to 20 μm,more preferably 1 to 10 μm.

To form a hard coat layer, it is preferably formed on the polyester filmhaving an adhesive layer by coating. The hard coat layer is preferablymade of a material having adhesion to both of the adhesive layer and thetransparent conductive layer, as exemplified by a mixture of a resincomponent such as acrylic resin, urethane-based resin, silicon-basedresin, UV curable resin or epoxy-based resin and inorganic particles. Asthe inorganic particles may be used alumina, silica or mica particles.

<Antireflection Layer>

In the present invention, an antireflection layer may be formed on theside opposite to the transparent conductive layer to increase lighttransmittance so as to improve photovoltaic power generation efficiency.

To form the antireflection layer, it is preferred that a single layer ortwo or more layers of a material having a refractive index differentfrom the refractive index of the polyester film should be formed. In thecase of a single-layer structure, a material having a smaller refractiveindex than that of the substrate film is preferably used whereas in thecase of a multi-layer structure consisting of two or more layers, alayer adjacent to the laminated film is preferably made of a materialhaving a larger refractive index than that of the polyester film and alayer formed on the above layer is preferably made of a material havinga smaller refractive index than that of the above layer.

The material constituting this antireflection layer may be an organic orinorganic material if it satisfies the above refractive indexrelationship. It is preferably a dielectric material selected from thegroup consisting of CaF₂, MgF₂, NaAlF₄, SiO₂, ThF₄, ZrO₂, Nd₂O₃, SnO₂,TiO₂, Ce, O₂, ZnS and In₂O₃.

To form the antireflection layer, dry coating techniques such as vacuumdeposition, sputtering, CVD and ion plating may be employed, or wetcoating techniques such as gravure coating, reverse coating and diecoating may be employed.

Prior to the formation of the antireflection layer, a pretreatment suchas corona discharge treatment, plasma treatment, sputter etchingtreatment, electron beam application, ultraviolet application, primertreatment or adhesion treatment may be carried out.

<Formation of Dye-Sensitized Solar Cell and Electrode for the Same>

To manufacture a dye-sensitized solar cell by using the electrode of thepresent invention, a known method may be employed. More specifically,the following method may be employed.

(1) A dye is adsorbed to the porous semiconductor layer of the laminatedfilm of the present invention. A dye having the property of absorbingvisible range light and infrared range light, such as an organic metalcomplex dye typified by a ruthenium bipyridine-based complex (rutheniumcomplex), cyanine-based dye, coumarine-based dye, xanthene-based dye orporphyrin-based dye is dissolved in a solvent such as an alcohol ortoluene to prepare a dye solution, and the porous semiconductor layer isimmersed in the dye solution, or sprayed or coated with the dye solutionto form one electrode A.(2) As a counter electrode, an electrode B manufactured by forming athin platinum layer on the transparent conductive layer side of thelaminated film of the present invention by sputtering is used.(3) The above electrodes A and B are joined together by inserting athermal pressure bonding polyethylene film frame spacer (thickness of 20μm) therebetween, and the spacer is heated at 120° C. to pressure bondthem together. Further, the edge portions are sealed up with an epoxyresin adhesive.(4) An electrolyte aqueous solution containing lithium iodide and iodine(molar ratio of 3:2) and 3 wt % of nylon beads having an averagediameter of 20 μm as a spacer is injected into the inside of the joinedproduct through small holes for injecting the electrolyte formed in acorner portion of the obtained sheet. The inside air is fully evacuated,and the small holes are closed with an epoxy resin adhesive in the end.

EXAMPLES

The following examples are provided to further illustrate the presentinvention. Evaluation items in the following examples and comparativeexamples were evaluated by the following methods.

(1) Particle Diameters of Crystalline Titanium Oxide Fine Particles andFiber Diameters of Crystalline Titanium Oxide Fibers

20 sites were selected at random from a photo of the surface of theobtained metal oxide taken by a scanning electron microscope (S-2400 ofHitachi Ltd.)(2,000 magnifications) to measure the diameters ofcrystalline titanium oxide fine particles and the fiber diameters ofcrystalline titanium oxide fibers so as to obtain their average valuesas the average diameter and the average length of the fine particles.

(2) Fiber Diameter/Fiber Length Ratio of Crystalline Titanium OxideFibers

The average fiber length and the average fiber diameter were calculatedby the same method as (1) that for measuring the particle diameters ofthe crystalline titanium oxide fine particles and the fiber diameters ofthe crystalline titanium oxide fibers so as to obtain their ratio.

(3) Method of Measuring BET Specific Surface Area

The specific surface area of the obtained metal oxide was measured by aBET method using a nitrogen gas.

(4) Measurement of X-Ray Diffraction

For the measurement of X-ray diffraction, a reflection method wasemployed by using the ROTA FLEX RU200B of Rigaku Denki Co., Ltd. and agoniometer having a radius of 185 nm, and CuKα ray monochromized with amonochrometer was used as the X-ray. A measurement sample obtained byadding high-purity silicon powders for X-ray diffraction standards as aninternal standard to the obtained ceramic fibers was used.

(5) Measurement of Crystallite Size

Intensity correction was made on the X-ray diffraction profile obtainedabove, and the diffraction angle 2θ was corrected with the 111diffraction peak of the internal standard silicon. The half-value widthof the 111 diffraction peak of silicon was 0.15° or less. Thecrystallite size was calculated from the following Scherrer's equationby using diffraction peaks which appeared at around 25.3° in thecorrected X-ray diffraction profile. The diffraction peaks of titaniumoxide and silicon at 2θ=24 to 30° were derived from Cu Kα1 and C Kα2rays, not separate from each other and all handled as being derived fromCu Kα.D=K×λ/β cos θD: crystallite size (nm)λ: measurement X-ray wavelength (nm)β: expansion of diffraction line by crystallite sizeθ: Bragg angle of diffraction peak (°)K: form factor (Scherrer's constant)

Since β corrects the expansion of an optical system, (β=B−b) obtained bysubtracting the half-value width “b” of the 111 diffraction peak of theinternal standard silicon from the half-value width “B” of thediffraction peak of titanium oxide which appeared at around 25.3° wasused, K=1 and λ=0.15418 nm.

(6) Anatase Phase Content Ratio Calculated from Integral Intensity Ratioof X-Ray Diffraction

The anatase phase content ratio calculated from the integral intensityratio of X-ray diffraction was obtained from the following equation byestimating integral intensity IA (anatase phase) and integral intensityIR (rutile phase) from diffraction peaks derived from anatase phase andrutile phase titanium oxides which appeared at 2θ=25.3° and 27.4°,respectively, in the X-ray profile obtained by carrying out intensitycorrection.Anatase phase content ratio=IA/(IA+IR)(7) Intrinsic Viscosity

The intrinsic viscosity ([η]) dl/g) was measured by using ano-chlorophenol solution at 35° C.

(8) Film Thickness

The thickness of the film at 300 locations in total was measured atintervals of 10 cm in the film forming direction and the width directionby using a micrometer (K-402B of Anritsu Co., Ltd.). The average valueof the obtained 300 film thickness data was calculated and taken as thefilm thickness.

(9) Light Transmittance

The light transmittance at wavelengths of 370 nm and 400 nm was measuredby using the MPC3100 spectrophotometer of Shimadzu Corporation.

(10) Thickness of Coating Layer

A small piece of the film was embedded in an epoxy resin (Epomount ofRefinetec Co., Ltd.), the resin was sliced to a thickness of 50 nm byusing the Microtome2050 of Reichert-Jung Co., Ltd., and the thickness ofthe coating layer was measured by observing through a transmissionelectron microscope (LEM-2000 of Topcon Corporation) at an accelerationvoltage of 100 KV and 100,000 magnifications.

(11) Surface Resistance Value

The surface resistance at 5 arbitrary points was measured by using afour-probe surface resistivity meter (Rolesta GP of Mitsubishi ChemicalCo., Ltd.), and the average value of the measurement data was taken as arepresentative value.

(12) I-V Characteristics (Light Current-Voltage Characteristics)

A 100 mm² dye-sensitized solar cell was formed to calculate photovoltaicpower generation efficiency by the following method. The solar simulator(PEC-L10) of Peccel Technologies Co., Ltd. was used to measurepseudo-sunlight having an incident light intensity of 100 mW/cm² in a25° C. and 50% RH atmosphere. A DC voltage applied to the system wasscanned at a constant speed of 10 mV/sec by using a current-voltagemeter (PECK 2400) to measure an optical current output from the elementso as to measure optical current-voltage characteristics and calculatephotovoltaic power generation efficiency.

Example 1 Formation of Polyester Film

A pellet of polyethylene-2,6-naphthalene dicarboxylate containingsubstantially no particles and having an intrinsic viscosity of 0.63 wasdried at 170° C. for 6 hours, supplied into the hopper of an extruder tobe molten at a melting temperature of 305° C., filtered with a stainlesssteel thin wire filter having an average opening of 17 μm, extruded ontoa rotary cooling drum having a surface temperature of 60° C. through a 3mm slit die and quenched to obtain an unstretched film. The unstretchedfilm obtained as described above was preheated at 120° C. and furtherheated with an IR heater set to 850° C. 15 mm from above to be stretchedto 3.1 times in the longitudinal direction between low-speed andhigh-speed rolls. The following coating A was applied to one side of thefilm obtained after stretching in the longitudinal direction with a rollcoater to ensure that the thickness of the coating film after drying was0.25 μm so as to form an adhesive layer.

The film was then supplied to a tenter to be stretched to 3.3 times inthe transverse direction at 140° C. The obtained biaxially oriented filmwas heat set at 245° C. for 5 seconds to obtain a polyester film havingan intrinsic viscosity of 0.58 dl/g and a thickness of 125 μm.Thereafter, this film was thermally relaxed at a relaxation rate of 0.7%and a temperature of 205° C. while it was suspended.

<Preparation of Coating A>

66 parts of dimethyl 2,6-naphthalenedicarboxylate, 47 parts of dimethylisophthalate, 8 parts of dimethyl 5-sodium sulfoisophthalate, 54 partsof ethylene glycol and 62 parts of diethylene glycol were fed to areactor, and 0.05 part of tetrabutoxy titanium was added to the mixtureand heated in a nitrogen atmosphere by controlling the temperature to230° C. to carry out an ester interchange reaction while the formedmethanol was distilled off. Then, the temperature of the reaction systemwas gradually raised to 255° C., and the inside pressure of the systemwas reduced to 1 mmHg to carry out a polycondensation reaction so as toobtain a polyester. 25 parts of this polyester was dissolved in 75 partsof tetrahydrofuran, 75 parts of water was added dropwise to the obtainedsolution under high-speed agitation at 10,000 rpm to obtain a milkywhite dispersion, this dispersion was distilled under a reduced pressureof 20 mmHg to remove tetrahydrofuran, and a water dispersion of thepolyester having a solid content of 25 wt % was obtained.

Then, 3 parts of sodium laurylsulfonate as a surfactant and 181 parts ofion exchange water were fed to a four-necked flask and heated up to 60°C. in a nitrogen gas stream, 0.5 part of ammonium persulfate and 0.2part of sodium hydrogen nitrite were added as polymerization initiators,and further a mixture of 30.1 parts of methyl methacrylate, 21.9 partsof 2-isopropenyl-2-oxazoline, 39.4 parts of polyethylene oxide (n=10)methacrylic acid and 8.6 parts of acrylamide all of which are monomerswas added dropwise over 3 hours while the temperature of the solutionwas adjusted to 60 to 70° C. After the end of the addition, the reactionwas continued under agitation while the above temperature was maintainedfor 2 hours, and then the reaction product was cooled to obtain anacrylic water dispersion having a solid content of 35 wt %.

Meanwhile, a water solution containing 0.2 wt % of a silica filler(average particle diameter: 100 nm) (Snowtex ZL of Nissan ChemicalIndustries, Ltd.) and 0.3 wt % of polyoxyethylene (n=7) lauryl ether(Naloacty N-70 of Sanyo Chemical Industries, Ltd.) as a wetting agentwas prepared.

8 parts by weight of the above water dispersion of the polyester, 7parts by weight of the acrylic water dispersion and 85 parts by weightof the water solution were mixed together to prepare a coating A.

<Hard Coat>

The obtained polyester film was used and a UV curable hard coating(Dezolite R7501 of JSR Corporation) was applied to the adhesive layerside of the polyester film to a thickness of about 5 μm and cured by UVto form a hard coat layer.

<Formation of Transparent Conductive Layer>

A transparent conductive layer made of IZO and having a thickness of 260nm was formed on the hard coat layer formed side by DC magnetronsputtering using an IZO target comprising indium oxide as the maincomponent and 10 wt % of zinc oxide. The formation of the transparentconductive layer by sputtering was carried out by evacuating the insideof a chamber to 5×10⁻⁴ Pa before plasma discharge, introducing argon andoxygen into the chamber to increase the inside pressure to 0.3 Pa andapplying electric power to the IZO target at a power density of 2 W/cm².The partial pressure of oxygen was 3.7 mPa. The surface resistance valueof the transparent conductive layer was 15Ω/□.

Subsequently, a plasma treatment was made on the surface of thetransparent conductive layer in a nitrogen gas stream (60 L/min) at arate of 1 m/min by using a normal pressure plasma surface treatmentdevice (AP-T03-L of Sekisui Chemical Co., Ltd.). At this point, thesurface resistance value was 16Ω/□ and the surface tension was 71.5mN/m.

<Antireflection Layer>

A Y₂O₃ layer having a thickness of 75 nm and a refractive index of 1.89,a TiO₂ layer having a thickness of 120 nm and a refractive index of 2.3and a SiO₂ layer having a thickness of 90 nm and a refractive index of1.46 were formed on the side opposite to the transparent conductivelayer side of the laminated film by high-frequency sputtering in thisorder to form an antireflection layer. When these electrostatic thinfilms were formed, the degree of vacuum was 1×10⁻³ Torr and 55 sccm ofAr and 5 sccm of O₂ were flown as gases. The substrate was kept at roomtemperature without heating or cooling in the course of forming thefilms.

<Formation of Crystalline Titanium Oxide Fibers by Electrospinning>

A solution containing 1 part by weight of polyacrylonitrile(manufactured by Wako Pure Chemical Industries, Ltd.) and 9 parts byweight of N,N-dimethylformamide (special grade, manufactured by WakoPure Chemical Industries, Ltd.) was mixed with a solution containing 1part by weight of titanium tetranormalbutoxide (first grade,manufactured by Wako Pure Chemical Industries, Ltd.) and 1 part byweight of acetylacetone (special grade, manufactured by Wako PureChemical Industries, Ltd.) to prepare a spinning solution. A fiberstructure was fabricated from this spinning solution by using theapparatus shown in FIG. 1. The inner diameter of a spray nozzle 1 was0.8 mm, the voltage was 15 kV, and the distance between the spray nozzle1 and an electrode 4 was 15 cm. That is, the solution 2 contained in asolution storage tank 3 was ejected from the spray nozzle 1 toward theelectrode 4. Meanwhile, a voltage of 15 kV was applied between theelectrode 4 and the spray nozzle 1 by a high-voltage generator. Theobtained fiber structure was heated up to 600° C. for 10 hours in an airatmosphere in an electric furnace and then kept at 600° C. for 2 hoursto manufacture a titania fiber. When the obtained titanium oxide havinga high aspect ratio was observed through an electron microscope, itsfiber diameter was 280 nm, its fiber length/fiber diameter ratio was 50or more, and the both ends of the fiber were not seen in the view fieldof a scanning electron microscope. The area ratio of the anatase phaseto the total of the anatase phase and the rutile phase in X-raydiffraction was 0.94. The anatase crystallite size was 22 nm. Accordingto the X-ray diffraction result of the obtained titania fiber, as asharp peak was seen at 2θ=25.3°, it was confirmed that the anatase phasewas formed. The BET specific surface area was 0.4 m²/g.

<Binder>

60 parts by weight of titanium tetraisopropoxide was added dropwise to120 parts by weight of 0.1 M nitric acid, and heated and refluxed for 12hours to be condensed so as to obtain a binder. The weight of the solidmatter after drying was 17 wt %.

<Formation of Porous Semiconductor Layer>

44 wt % based on the total weight of all the titanium oxides of theabove crystalline titanium oxide fibers, 44 wt % based on the totalweight of all the titanium oxides of the SP-200 titanium oxidedispersion (content of titanium oxide: 25 wt % anatase phase and a smallamount of rutile phase) of Showa Titanium Co., Ltd. as the crystallinetitanium oxide fine particles and 12 wt % based on the total weight ofall the titanium oxides of the above binder were dispersed into ethanol(manufactured by Wako Pure Chemical Industries, Ltd.) to prepare adispersion having a solid content of 12 wt % and treat it by theapplication of 40.0 Hz ultrasonic waves for 30 minutes. As a result, asolution for the porous semiconductor layer was obtained. This solutionwas applied to the transparent conductive layer with a bar coaterimmediately and heated at 180° C. in the atmosphere for 5 minutes toform a 5 μm-thick porous semiconductor layer. The peel-off andbrittleness of the porous semiconductor layer were not seen after theheat treatment, and an electrode for dye-sensitized solar cells havinghigh adhesion to the substrate was manufactured.

When the X-ray diffraction of the porous semiconductor layer obtained asdescribed above was carried out, a peak derived from the anatase phaseand a weak peak derived from the rutile phase were seen, the anatasephase content ratio calculated from the integral intensity ratio ofX-ray diffraction was 0.92, and the size of the anatase phase crystalwas 24 nm.

<Fabrication of Dye-Sensitized Solar Cell>

This electrode was immersed in a 300 μM ethanol solution of a rutheniumcomplex (Ru535bisTBA of Solaronix Co., Ltd.) for 24 hours to adsorb theruthenium complex to the surface of the optical function electrode. A Ptfilm was deposited on the transparent conductive layer of theabove-described laminated film by sputtering to form a counterelectrode. The electrode and the counter electrode were joined togetherwith a thermal pressure bonding polyethylene film frame spacer(thickness of 20 μm) therebetween, and the spacer was heated at 120° C.to pressure bond these electrodes together. Further, the edge portionsof these electrodes were sealed up with an epoxy resin adhesive. Afteran electrolyte solution (3-methoxypropionitrile solution containing 0.5M lithium iodide, 0.05 M iodine and 0.5 M tert-butylpyridine) wasinjected into the inside of the obtained product, the product was sealedup with an epoxy-based adhesive.

When the I-V characteristics of the completed dye-sensitized solar cellwere measured (effective area of 100 mm²), the open voltage,short-circuit current density and curve factor were 0.70 V, 8.25 mA/cm²and 0.47, respectively. As a result, photovoltaic power generationefficiency was 2.71%.

Example 2

The procedure of Example 1 was repeated except that the AMT-100 titaniumoxide for optical catalysts of Teika Co., Ltd. (average particlediameter: 6 nm, anatase phase) was used as crystalline titanium oxidefine particles used to form the porous semiconductor layer. Thecharacteristic properties of the titanium oxides are shown in Table 1.The characteristic properties of the obtained porous semiconductor layerand the evaluation results of the obtained cell are shown in Table 2.

Example 3

The procedure of Example 1 was repeated except that 0.5 part by weightof titanium tetranormalbutoxide (first grade, manufactured by Wako PureChemical Industries, Ltd.) was used to form crystalline titanium oxidefibers. The characteristic properties of the titanium oxides are shownin Table 1. the characteristic properties of the obtained poroussemiconductor layer and the evaluation results of the obtained cell areshown in Table 2.

Example 4

The procedure of Example 1 was repeated except for the formation of thecrystalline titanium oxide fibers and the formation of the poroussemiconductor layer.

<Formation of Crystalline Titanium Oxide Fibers by Electrospinning>

1.3 parts by weight of acetic acid (special grade, manufactured by WakoPure Chemical Industries, Ltd.) was added to 1 part by weight oftitanium tetranormalbutoxide (first grade, manufactured by Wako PureChemical Industries, Ltd.) to obtain a homogeneous solution. A gel wasformed in the solution by adding 1 part by weight of ion exchange waterto this solution under agitation. The formed gel was dissociated byfurther continuing agitation so that a transparent solution could beprepared.

0.016 part by weight of polyethylene glycol (manufactured by Wako PureChemical Industries, Ltd., first grade, average molecular weight of300,000 to 500,000) was mixed with the prepared solution to prepare aspinning solution. When spinning was carried out from this spinningsolution by using the apparatus shown in FIG. 1, a planar fiberstructure was obtained on the electrode 4. The inner diameter of thespray nozzle 1 was 0.4 mm, the voltage was 15 kV, and the distancebetween the spray nozzle 1 and the electrode 4 was 10 cm. A deposit ofcrystalline titanium oxide fibers in a nonwoven state was formed byheating the obtained fiber structure up to 600° C. in an air atmospherein an electric furnace for 10 hours and keeping the fiber structure at600° C. for 2 hours. The characteristic properties of the crystallinetitanium oxide fibers obtained as described above are shown in Table 1.

<Formation of Porous Semiconductor Layer>

The above-described crystalline titanium oxide fibers in a nonwovenstate (8.1 g/m²), 43.5 wt % based on the total weight of all thetitanium oxides of the SP-200 titanium oxide dispersion (content oftitanium oxide: 25.1 wt % anatase phase and a small amount of rutilephase) of Showa Titanium Co., Ltd. as the crystalline titanium oxidefine particles and 13 wt % based on the total weight of all the titaniumoxides of the above binder were applied to the transparent conductivelayer and heated at 180° C. in the atmosphere for 5 minutes to form a 5μm-thick porous semiconductor layer. The peel-off and brittleness of theporous semiconductor layer were not seen after the heat treatment, andan electrode for dye-sensitized solar cells having high adhesion to thesubstrate was manufactured. The characteristic properties of thecrystalline titanium oxide fibers obtained as described above are shownin Table 2.

A dye-sensitized solar cell was manufactured in the same manner as inExample 1 by using the porous semiconductor obtained as described above.The evaluation result of cell performance is shown in Table 2.

Example 5

A porous semiconductor layer was formed in the same manner as in Example1 except that crystalline titanium oxide fibers were manufactured in thesame manner as in Example 4. The characteristic properties of thecrystalline titanium oxides are shown in Table 1 and the characteristicproperties of the porous semiconductor layer are shown in Table 2. Adye-sensitized solar cell was manufactured in the same manner as inExample 1 except that the porous semiconductor obtained as describedabove was used. The evaluation result of cell performance is shown inTable 2.

Comparative Example 1

A porous semiconductor layer was obtained in the same manner as inExample 1 except that crystalline titanium oxide fine particles were notadded at the time of forming the porous semiconductor layer, and adye-sensitized solar cell comprising the same was evaluated. Ashort-circuit current dropped due to the addition of no fine particleswith the result of a reduction in photoelectric conversion efficiency.

Comparative Example 2

A porous semiconductor layer was obtained in the same manner as inExample 5 except that crystalline titanium oxide fine particles were notadded at the time of forming the porous semiconductor layer, and adye-sensitized solar cell comprising the same was evaluated. The resultsare shown in Table 2.

Comparative Example 3

Although a porous semiconductor layer was obtained in the same manner asin Example 1 except that crystalline titanium oxide fibers were notadded at the time of forming the porous semiconductor layer, the partialpeel-off of the porous semiconductor layer was seen. A dye-sensitizedsolar cell comprising the same was evaluated. The results are shown inTable 2.

TABLE 1 Crystalline titanium Crystalline titanium oxide fibers oxidefine Crystal BET particles Fiber Fiber phase Anatase specific Particlediameter length/fiber area crystal surface Amount diameter Amount (nm)diameter ratio size (nm) area (m²/g) (wt %) (nm) (wt %) Example 1 28050< 0.94 22 49.0 44 46 44 Example 2 280 50< 0.94 22 49.0 44 10 44Example 3 172 50< 0.98 16 65.2 44 46 44 Example 4 284 50< 1.00 160 0.344 46 44 Example 5 284 50< 1.00 160 0.3 44 46 44 Comparative 280 50<0.94 22 49.0 87 — — Example 1 Comparative 284 50< 1.00 160 0.3 87 — —Example 2 Comparative — — — — — — 46 87 Example 3

TABLE 2 Porous semiconductor Anatase phase Anatase Evaluation of cellcontent crystal Jsc Crystal peak ratio size (nm) Voc (V) (mA/cm²) FF Eff(%) Example 1 anatase.rutile 0.92 24 0.70 8.25 0.47 2.71 Example 2anatase.rutile 0.95 19 0.68 7.00 0.44 2.09 Example 3 anatase.rutile 0.9621 0.70 8.80 0.45 2.77 Example 4 anatase.rutile 0.94 95 0.71 6.18 0.461.99 Example 5 anatase.rutile 0.95 93 0.71 7.29 0.49 2.54 Comparativeanatase.rutile 0.93 23 0.69 2.95 0.40 0.78 Example 1 Comparative anatase1.00 150 0.66 0.21 0.60 0.10 Example 2 Comparative anatase.rutile 0.9230 0.71 3.41 0.46 1.59 Example 3 Voc: open voltage Jsc: short-circuitcurrent FF: fill factor Eff: efficiency

The invention claimed is:
 1. A laminated film comprising a poroussemiconductor layer, a transparent conductive layer and a transparentplastic film, wherein the porous semiconductor layer comprisescrystalline titanium oxide fibers having a fiber diameter of 50 to 1,000nm and a fiber length/fiber diameter ratio of 5 or more and being in anon-woven state in an amount of 10 wt % or more and crystalline titaniumoxide fine particles having a particle diameter of 2 to 46 nm in anamount of 15 wt % or more, the crystalline titanium oxide fibers and thecrystalline titanium oxide fine particles are substantially composed ofan anatase phase and a rutile phase, the anatase phase content ratiocalculated from the integral intensity ratio of X-ray diffraction isbetween 1.00 and 0.32, and the laminated film is used in an electrodefor dye-sensitized solar cells.
 2. The laminated film according to claim1, wherein an average crystallite size measured by the X-ray diffractionof the anatase phase of the porous semiconductor layer is 10 to 100 nm.3. An electrode for dye-sensitized solar cells which comprises thelaminated film of claim 1 and a dye adsorbed to the porous semiconductorlayer of the laminated film.
 4. A dye-sensitized solar cell comprisingthe electrode of claim
 3. 5. The laminated film according to claim 1,wherein the transparent plastic film is of polyethylene-2,6-naphthalate.